1. Field of the Invention
The present invention relates to an alternating current (AC) light-emitting diode (LED) driving circuit and more particularly to a current-shunting AC LED driving circuit adapted to conventional AC power sockets and mitigating the high-temperature drawback.
2. Description of the Related Art
LED is one of the most common lighting devices in the current market. In contrast to conventional incandescent bulbs, LED is characterized by high lighting efficiency and economical power consumption. Due to the nature of allowing current to flow in only one direction, LED is hard to receive power from conventional AC power sockets. In this regard, LED manufacturers have developed an AC LED driving circuit. With reference to FIG. 5, the AC LED driving circuit has a rectification unit 20, an LED unit 21, a voltage-controlled transistor 22, a current detection unit 23, a low-pass filter 24 and a steady current control unit 25.
The rectification unit 20 has an input terminal connected to an AC power source, converts the AC power into a pulsed DC power and outputs the pulsed DC power.
The LED unit 21 has multiple LEDs and electrically connected to an output terminal of the rectification unit 20 to constitute a power loop.
The voltage-controlled transistor 22 is serially connected to the power loop and has a control terminal for adjusting a loop current of the power loop.
The current detection unit 23 is serially connected to the power loop to convert the loop current of the power loop into a corresponding voltage signal.
The low-pass filter 24 is electrically connected to the current detection unit 23 and outputs an average voltage according to the voltage signal converted by the current detection unit 23.
The steady current control unit 25 has an input terminal electrically connected to the low-pass filter 24, another input terminal electrically connected to a reference voltage and an output terminal electrically connected to the control terminal of the voltage-controlled transistor 22. The steady current control unit 25 compares the reference voltage value received by the input terminal thereof with the average voltage value, and outputs a control signal based on the comparison result to stabilize the loop current of the power loop.
The rectification unit 20 serves to convert the inapplicable AC power into the pulsed DC power. The current detection unit 23 and the low-pass filter 24 detect the average value of the loop current flowing through the LED unit 21. The steady current control unit 25 controls the loop current adjusted by the voltage-controlled transistor 22 to supply power to the LED unit 21 for sustaining uniform lighting.
The voltage-controlled transistor 22 is serially connected to the overall power loop so as to control the loop current. Hence, the loop current completely flows through the voltage-controlled transistor 22 to cause constant high power consumed by the voltage-controlled transistor 22. When normally operated, the voltage-controlled transistor 22 needs to be stably operated at a current of 0.16 amps and withstand a voltage in a range of 5˜25 volts. Standardized by the normal operation condition, the voltage-controlled transistor should be subject to a power in a range of 0.8˜4.0 W. Such power range overwhelms any transistor in the market without exception and introduces extraordinary load to an integrated circuit especially when the voltage-controlled transistor, the low-pass filter and the steady current control unit are integrated in the integrated circuit. When the voltage-controlled transistor is subject to the power in the range of 0.8˜4.0 W and a temperature of the voltage-controlled transistor suddenly rises to 150° C. within few minutes, the integrated circuit with the voltage-controlled transistor thereon surely fails to be operated normally.